Multiple-element telescopes have been utilized in the past to reshape the output of pump lasers so that the pump laser output can be more efficiently coupled, for instance, to an optical parametric oscillator (OPO). The purpose of the telescope is to be able to magnify or demagnify the pump laser output so as to duplicate the waist of the beam which exists at the output mirror of the pumping laser and position it at a precise location within the nonlinear crystal utilized in the optical parametric oscillator. As a result, for a given diameter waist associated with an output beam, and with the waist existing at the planner output mirror of the pumping laser, it is important to be able to either magnify or demagnify the waist so as to provide a predetermined waist within the body of the nonlinear crystal.
In general, for optical parametric oscillators, it is important to be able to demagnify the beam produced by the pump laser so that the telescope is in essence a demagnifying telescope. Typically, for a 1.06 micron pump laser, in certain applications it is important to have a 0.5× telescope so that if the waist diameter, for instance, at the pump laser is 0.5 millimeters, then the waist diameter within the optical parametric oscillator crystal is to be 0.25 millimeters. To do this, the characteristics of the pumping laser output within the optical parametric oscillator must be carefully controlled.
More specifically, it is important to control aberrations caused by the telescope and most importantly third-order aberrations so as to limit the waist diameter. Typically, third order aberrations cause waists which are larger than desired and therefore materially affect the efficiency of the optical parametric oscillator. Oftentimes aberrations cause a 10% decrease in efficiency in the optical parametric oscillator or sometime as much as a 50% decrease. This is unacceptable and multi-element telescopes have to be carefully designed to minimize aberrations.
Moreover, the inability to control the waist within the optical parametric oscillator crystal can result in a waist which is considerably smaller than that for which the system is designed. When such third-order aberrations occur, pumping with a waist too small can in fact drill a hole completely through the nonlinear crystal in the OPO. Most usually for reduced-diameter waists, the concentration of energy within the nonlinear crystal can cause the nonlinear crystal to crack, thus causing the laser to malfunction or die.
There is also a considerable ghost problem when utilizing multi-element telescopes. The ghost occurs by back reflection into the laser cavity of the pumping laser, which typically produces nanosecond Q-switched pulses. Note that the retro-reflective ghost can damage the Q-switch or can cause pre-lasing in the pump laser.
Antireflective coatings are utilized in part to limit the amount of retro-reflective ghost that occurs. However, antireflective coatings can only reduce ghosts so far. As will be appreciated, the larger the number of lenses there are, the worse are the ghost retro-reflections. This is because back reflection into the laser cavity of the pumping laser occurs from the reflective surfaces the lenses used in the multi-element set. For instance, in a four-element telescope, there are eight reflective surfaces which must be taking into account. Some multi-element telescopes utilize six or seven lenses, resulting in 12 to 14 reflective surfaces.
Antireflective coatings on these surfaces are designated, for instance, to be 99.5% transmissive and 0.5% reflective. However, even with back reflection limited to 0.5%, deleterious ghosts occur. The problem of eliminating ghosts is complicated with multi-element sets due to the fact that while ghosts are less important when a convex lens surface points back to the pump laser which results in a diverging back reflection, lens surfaces which are concave when pointed at the pumping laser result in hot spots of back-reflected energy.
Of course, the utilization of multiple lenses in a telescope requires very accurate calibration procedures and ones which take into account the types of materials utilized in the lenses, their temperature coefficients of expansion, their refractive indices, and indeed the stability of the optical bench on which the lenses are mounted. Note that with temperature changes, the physical dimensions of the optical bench change, thus causing alignment errors. This is compounded in proportion to the number of optical elements mounted to the optical bench.
Thus, increasing the complexity of the telescope increases the retro-reflected ghosts that can cause pre-lasing as well as presenting alignment complexities that are hard to compensate for.
Moreover, for a typical three-element telescope, the cost may be on the order of $2,500.00, which is indeed a large cost factor in the overall cost of the laser system. When optical parametric oscillators are utilized for tunable mid-IR radiation generation for use, for instance, in countermeasures, laser target designators, or atmospheric pollution measurements, the cost of the telescope is sometimes cost-prohibitive.
Additionally, if third-order aberrations are caused by the multi-element set, then as mentioned before efficiencies of the overall laser system may be reduced by as much as 50%, especially when the waist provided within the nonlinear crystal is larger than desired.
Thus, there are a wide variety of problems associated with multi-element telescopes utilized to reshape the output beam of a pumping laser, including the lack of ability to control the waist in the optical parametric oscillator, the inability to control retro-reflecting ghosts, the inability to properly calibrate the multi-element set of lenses, and indeed the overall cost of the multi-element set.